How can I cast a text element to float in animation-nodes?
Search in the menu (Shift+A) for Convert

It's a generic node that converts all to all. 
There are other nodes that are not in the menu yet.



Sometimes it is added automatically.

In this case it is not cause you have to be a bit careful about the input, not to get a None. None is not taken well by some types of data/sockets and will give errors for vectors and others. So don't jump in using it all around :)